This invention relates to a method and apparatus for measuring stress in an object. More particularly, this invention relates to a method and apparatus for measuring stress in an object by non-destructive means using ultrasonic waves.
Stress measurements are useful in determining the structural integrity and safe service life of manufactured articles. If a particular article still has useful life, it would be desirable to evaluate its stress by non-destructive means.
Several non-destructive stress evaluation techniques are known. One well-developed technique involves X-ray diffraction, in which X-rays are used to measure the distance between planes of atoms in the material being evaluated. Displacement of the planes from their normal position indicates the presence of stress in the material. Application of this method is limited by the inability of X-rays to probe deeper than about a thousand atomic layers into the material, and by the method's total inapplicability to non-crystalline materials.
Other methods of determining stress involve ultrasonic waves. All ultrasonic methods depend in principle upon the fact that the velocity of propagation of ultrasonic wave in a solid medium is influenced by stresses present in the medium. Although the effect is small, its detection and measurement are within the present state of the ultrasonic art. However, the velocity of ultrasonic waves is also affected by microstructural variations and anisotropies. Therefore, the determination of the velocity of a single ultrasonic wave alone cannot give an accurate indication of stress in a material.
It would thus be desirable to be able to measure stress in a material by ultrasonic means independent of anisotropies and microstructural variations. A theoretical basis for such measurements has been developed in the literature. In a rigorous theoretical investigation of the influence of initial stress on elastic ultrasonic waves, it was noted that the propagation of elastic waves in a material under initial stress is fundamentally different from the stress-free case, and follows laws which cannot be explained by elastic anisotropy or changes in elastic constants. Biot, Jr., Maurice A., Applied Physics, 11, 522 (1940). It has also been noted that an ultrasonic shear wave propagates faster in the direction of tension than in the perpendicular direction, analogous to a wave in a stretched string, and that the difference in .rho.V.sup.2 for the two waves is equal to the tensile stress, where .rho. is the material density and V is shear wave velocity. Thuston, R. N., J. Acoustical Society of America, 37, 348 (1965). It has been suggested that the effects of stress and texture can be separated by comparing ultrasonic wave velocities, MacDonald, Douglas E., IEEE Transactions on Sonics and Ultrasonics, SU-28 75 (1981). despite this extensive theoretical development, there has heretofore been no method or apparatus by which to apply this theory to actual stress measurements.
Previously known ultrasonic techniques have involved the use of vertically polarized or vertically propagated shear waves. For example, in the method known as shear wave birefringence, two shear waves are generated which are propagated in the same vertical direction but whose directions of polarization are orthogonal to one another. The difference in velocity between the two waves is an indication of the stress in the material. The birefringence technique is nevertheless subject to inaccuracies due to velocity shifts caused by preferred orientation of elastically anisotropic grains.
Another known method of stress measurement is set forth in U.S. Pat. No. 4,080,836 to Thompson, et al. In this method, an electromagnetic acoustic transducer generates vertically propagating orthogonally polarized shear waves in a material. The difference in velocity between the polarized waves is measured and compared to a known correlation between the difference in velocity of orthogonally polarized shear waves in the type material being measured and stress in the material. In this way the stress in the particular article is obtained. However the utility of this method is inherently limited by the accuracy of the known correlation.